Treatment of speech waves for transmission or recording



June 1942- w. DUDLEY 2,286,072

TREATMENT OF SPEECH WAVES FOR TRANSMISSION OR RECORDING Filed Dec. 22,1939 s Sheets-Sheet 1 I "(b o comm yous Q Mar/aw an r. near Man mm ommmmr a o cowmvww o R a mar/01v MOD M PER/0D I wvnvrop H. W DUDLEYATTORNEY June 9, 1942. H. w. DUDLEY TREATMENT OF SPEECH WAVES FORTRANSMISSION OR RECORDING s Sheets-Sheet. 2

Filed Dec. 22, 1939 LONE INTERVAL i go/wasp 9 NET CHANNEL CHANNEL 2 NETINVENTOR H. W DUDL E X By ATTORNEY June 9, 1942.

H. w. DUDLEY 2,286,072 TREATMENT OF SPEECH WAVES FOR TRANSMISSION 0RRECORTDING Filed Dec. 22, 1939 s Sheets-Sheet 3 FIG. 5

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ALTEfi/VA TE CIRCUI T F OR SECOND MES 54 GE FIG. 6A

INVENTOR h. n! DUDLEY ATTORNEY Jane 2, i942 TREATMENT OF SPEECH WAVESIFQJR TNSIVHSSION R RECORD-ENG Homer W. Dudley, Garden City, N. Y2,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application December 22, 1939, Serial No.310,527

18 Claims.

The present invention relates to transmission of speech or other messagewaves in which the waves are chopped and only certain'fragments aretransmitted.

A general object of the invention is to improve the quality orintelligibility of the reproduced The various more specific objects aswell as the nature of the invention itself will be made clear in thedetailed description to follow in connection with the drawings in which:

Figs. 1 and 2 are typical wave forms to be referred to;

Fig. 3 is a schematic circuit diagram of one terminal of a multiplexsystem according to the invention;

Fig, 4 is a similar diagram of a terminal of an alternative type ofmultiplex system according to the invention;

Fig. 5 is a similar diagram of one terminal of a two-way privacy systemembodying the invention; and

Figs. 6, 6A and 7 show circuits for recording and reproducing dictatedmaterial, according to the invention.

Various proposals have been made in the past for subdividing or choppingspeech waves on a time basis, discarding certain of the subdividedfragments, such as every other one, and transmitting only theintervening fragments. Various rates of chopping have been proposed froma few per second up to several hundred or more per second. The presentapplicant in his United States Patent 2,115,803, May 3, 1938, discloseda system in which the rate of chopping is the same as the fundamental orvocal cord frequency in the case of the voiced sounds and is of the sameorder of magnitude in the case of unvoiced sounds. The rate of choppingis placed under the control of the voice waves so that the rate isdefinitely related to and varies with the fundamental voice frequency.The reason for this-is given in the patent and is found in the nature ofspeech itself as illustrated in typical sound wave patterns shown by wayof example in Figs. 1 and 2. I

Figs. 1 and 2 are respectively taken from plate No. 160 and plate No. 41Man article, "The Sounds of Speech, by Irving B. Crandall, in the BellSystem Technical Journal, vol. IV, page 586, 1925.

Fig. 1 shows the sound s in part.This is made by placing the tip of thetongue against the Crandalls plate No. 41.

point. The outcoming air forms eddies or whirling currents at thispoint, giving rise to the sound. As these eddies are random inamplitude, frequency of occurrence, and location, they do not give anypattern repeating itself periodic-ally. Instead they give a fluctuatingcurrent that remains essentially the same over long periods of time.This can be seen from the figure.

Fig. 2 shows one period of the voiced sound a as in father beginning attime .064 second on The fundamental frequency is about cycles persecond. It is seen that at the beginning of the period there are strongamplitudes and sharp variations, while at the end there are weakamplitudes and less sharp variations. It may be considered that theperiod starts with a puff of air which traverses an acoustical resonantsystem with the energy damping out and trailing off near the end of theperiod.

In the method and system of transmission disclosed in the patent it isproposed to delete alternate segments of fundamental period length orduration and to supply these again at a receiving point by repetition.The present invention is based upon a different principle which can bemade apparent from aconsideration of the voice sound traces shown inFigs. 1 and 2. Upon examining one period of the voiced "a sound, onenotices that the amplitude swings at the middle of the sound have.dropped to a small fraction of what they were at the beginning of thesound. For exampleythe amplitude at the middle point of the fundamentalperiod in the case of a vowel may be only about one-tenth what it wasinitially. This means that the energy is only one one-hundredth asgreat, and so in comparison is negligible. This is particularly truesince the ear cannot adjust its sensitivity over so small a period oftime. Moreover, the component frequencies upon analysis are seen toexhibit heavy damping factors. This is to be expected from the manner,indicated above, in which the sound is produced, starting with a puff ofenergy which sets up vibrations in the resonant system that are rapidlyclamped to low amplitude before the next puff of energy occurs.

According to the present invention in its preferred form, the waves arechopped in such a way as to retain the large energy part of the voicedsound and to delete the low energy portion. For unvoiced sounds,illustrated. in Fig. 1, corresponding segments are retained'andsuppressed in each period of fundamental length chosen somewhatarbitrarily to be about the length of thevoiced periods. This means thatthe rate of interruption has to be related to the fundamental frequencyof the voice since otherwise the dividing line between retained and suppressed segments would not fall between the high energy and low energyportions of each fundamental period of the voiced waves. In actual'speech the vocal cord rate or fundamental frequency is continuallyvarying. It is this that gives inflection, emphasis and shades ofmeaning by varying intonations. A fixed rate of interruption of anyarbitrary value would not make the points of interruption coincide withparticular points in the fundamental period. The distortion resultingfromcutting the speech in this manner will be small in view of thelowenergy level of the portion eliminated. It is also important that thecutting be done at the fundamental frequency rate. If this is not done,the cutting frequency and the vocal cord frequency beat together toproduce raucous sounds that are highly unpleasant to the ear. It isfound in practice that a large amount of the energy in the fundamentalperiod can be cut out without offending the ear provided the rate ofcutting is tuned to' the vocal cord rate.

The fundamental frequency of the sound pictured in Fig. 2 is about 100cyclesper second. Crandall found a mean value of 125 cycles per secondfundamental frequency for male speakers used in his experiments and amean value of 244 cycles per second fundamental frequency for the femalespeakers or approximately twice as high as that of the male speakers. Heand others find that in pronouncing a vowel sound the speaker varies thefundamental pitch several per cent from the average, for example, 25 percent or even more.

Since most of the energy in speech is contained detailed disclosure thatone half of each fundamental length wave is suppressed and that a secondmessage or wave is transmitted in the time intervals thus saved. Byextension, however, it will be obvious how additional speech messagescan be sent when the longer interruptions referred to are used..

Referring to Fig. 3, there are shown at the 1 left two ordinarytelephone lines L1 and L2 arranged for simultaneous transmission to orreception from the multiplex line ML shown at the right. This is madepossible by the fact that each of the message waves on lines L1 and L2is interrupted for one half the time in transmission through theequipment shown in this figure, and the time thus saved is utilized fora second message thus permitting the duplex transmission.

Considering the equipment associated with line L1, this line isterminated in the usual hybrid coil H1 and balancing network N1. Thetransmitter is shown enclosed in the broken line T1. The transmittingbranch from the hybrid coil H1 leads to the subdivider circuit shown indotted line enclosure 9, including rectifier l0 and bandpass filter llof gradual cut-off which together serve to derivethe fundamental orvocal cord motor 93 which is driven with an intermittent in the voicedsounds, suppression of only the low energy portions of each fundamentalperiod wave of the voiced sounds and a corresponding fraction of theunvoiced sounds will make but very little diiferencein the total energytransmitted, even though this results in reducing the energy of theunvoiced sounds by half or two thirds, these sounds having such smallenergy content. The saving in time is, therefore, made without much lossin energy transmitted. Applicant has found that the loss inintelligibility from suppressing the low energy half of each fundamentallength wave may be very small, since the ear response isdue almostentirely to the high ener y portion of the fundamental length wave, the

motion (indicated by the impulse diagram l3) by the rectified impulsesfrom the output of filter H, the motor either not running at all or elserunning at a uniform speed. This motor l3 drives the magnetic tape ISwith an intermittent motion toward the right. For simplicity the motoris indicated as starting and stopping at the fundamental rate. Obviouslythe motor may run continuously and may drive the tape through a clutchintermittently operated, one form being shown in my patent abovereferred to. When there is an interruption in the receipt of voicedsounds the output of filter H ceases and relay I 2 releases and connectsa source M to the rectifier 25 and motor F3 to continue driving theeffect of the low energy portion being a blank so far as the earresponse is concerned. This fact makes the time interval represented bythe low energy portion available for other uses as will be described.

The invention is not limited to any definite fraction as the fraction ofthe fundamental length period that is to be suppressed. This may be onehalf, two thirds or three fourths or any suitable or permissible valuedepending upon the conditions obtaining in a'nyparticular case. If

7 sufficient intelligibility is obtainable to meet the requlrementsofany given case by interrupting for three fourths of the fundamentalperiod, as many as four speech messages can be sent in the time andfrequency limits occupied bya single speech message inordinarytelephony. If two thirds of the fundamental period wave can besuppressed in a given case, three speech messages can be sent withinthese same time and frequency limits. For simplicity of illustration anddescription, it will be assumed in the present motor intermittently at arate which corresponds to a mean fundamental frequency for voicedsounds. This is in order that the unvoiced sounds may also beinterrupted one half of the time.

The speech waves themselves pass through the delay network 24 of thesubdivider circuit and amplifier IE to the recording magnet 11, the coreof which is near or in contact with the tape l5. During movement of tapel5 underneath the core of magnet I! a magnetic record is produced of thereceived sound waves. When the tape is stationary the recorder isineffective, relay 26 at its back contact shunting the recording magnetll between times of movement of the tape.

It is seen from the above description that the record made on tape I5 isa continuous record composed of segments of speech waves one half afundamental period in length and lying adjacent one another with theintervening portions of the received waves dropped out.

The tape passes on to the right and the portion W of the tape is given aconstant motion by motor l9 driven from a suitable batterv with suitablespeed control. This speed can be set at a fixed value which is one halfthe speed of movement of the tape at position 15 when the latter ismoving. Beading magnet 20 picks up the recorded speech and reduces thefrequency range by a factor of 2. These reproduced waves are transmittedthrough amplifier 2! to hybrid Ha and then through low-pass filter 22 tothe line from receiving branch 23 is sent through amplifier 28 to thewinding of recording magnet 29 so that a record is made on the tape 30corresponding to the record sent from a tape corresponding to tape IIIat the distant terminal.

Some of the received speech is rectified at 3| and filtered at 32 toderive the fundamental frequency. It is observed that since the tramitting tape 18 moves at half speed, a hi 11 energy portion of therecorded wave representing a voiced sound passes under the readingmagnet once in each fundamental cycle. Accordingly, the fundamentalfrequency is readily derived in the receiver by rectifying and filteringat 3| and 32 and the relay 33 is operated in the same manner as relay l2at the sending end. The fundamental frequency impulses rectified at 25'drive motor 34 at an intermittent fundamental rate (as indicated by thediagram 34'), thus causing the tape portion 36 to move toward the leftintermittently in short jumps. The speed of movement of the tape portion36 when it is in motion is twice the speed of the tape portion 30.

The waves picked up in the coil 35, therefore,-

have their frequency range increased by a factor of 2 with respect tothe frequency range of the waves in recorder 29. These waves in readingmagnet 35 consist of segments of half funda-.

mental period duration spaced apart by intervals, of the same length, ofno current. These waves are sent to amplifier 31 through hybrid coil Hito the line L1. When there are no voiced waves being received, relay 33drops back and connects generator 35' to the rectifier 25" and motor 36.Generator 35' runs synchronously with the generator corresponding to thegenerator M at the opposite terminal. The tape portion 33 is thereforedriven ahead intermittently in jumps to enable reproduction of unvoicedsounds recorded thereon.

As noted above, the operation of the terminal apparatus for line L1transmits and receives one speech message over the line ML in one halfthe frequency range required by the methods of ordinary telephony.

The remaining half of this frequency range is used by the terminalapparatus of the line La for transmitting and receiving a second speechmessage. For this purpose the transmitter T2, indicated by broken linerectangle, may be the same as transmitter T1 and the receiver R2,indicated by broken line rectangle, may be the same as receiver R1.

Waves from line L2 transmitted through transmitter T2 pass intomodulator which may be of from source it of proper value to modulatewith 1' the output of T2 and raise each of the frequencies by a fixedamount, for example, approximately 1,500 cycles, or to give a reverseside-band, so that they occupy the upper half of the ordinary commercialtelephone speech band. These waves are then transmitted through thehigh-pass filter M to the line ML.

Waves from the distant terminal produced in similar manner and occupyingthe same frequency range are received through the high-pass filter 66and sent into the demodulator 42 where they modulate with the continuouswave from source All and are stepped down in frequency to the frequencylevel at which they were produced by the transmitter at the distantstation corresponding to transmitter T2. These waves then pass throughthe low-pass filter 43 and into the receiver R: where they aretranslated into segments of speech waves each of half fundamentalduration as in the case of receiver R1 and are sent into the line L2.

In the system that has just been described, the saving in time resultingfrom discarding one half of each wave portion of fundamental length isconverted into a saving in frequency by reducval not used by the firstmessage. One way in which this may be accomplished is shown in Fig. 4which will now be;described.

In Fig. 4,, lines L1 and L2 are arranged for twoway repeating with themain line ML for securing the transmission over the latter line of twospeech messages within the frequency and time limits required by asingle speech message when transmitted by ordinary telephony. Referringfirst to the apparatus of channel I associated with line L1, the speechis recorded on the magnetic tape l5 in the same manner as described inconnection with Fig. 3, the corresponding appa ratus elements beingdesignated by the same reference characters as'in Fig. 3. The wavespicked up from the portion l8 of the tape bythe reading magnet 20 are inthis case, however, transmitted to line without any frequency reduction.For this purpose the tape is driven by motor IQ for a certain length oftime as determined by interrupter 52 and the speed of motion of the tapeportion I8 past the reading magnet is the same as the speed of movementof tape portion I5 during the times when the tape portion is moved. Thecontinuously rotating interrupter b2 controls the energization of tworelays 50 and El each of which is energized half the time anddeenergized half the time. The period of the interrupter 52 is such asto allow a considerable number of recorded half segments of speech offundamental duration to be transmitted from the tape it withoutinterruption. When the brush of the interrupter 52 is on its insulatingsegment relays 5t and at are de-- energized. Relay 5i closes the drivingcircuit for the motor iii to move the tape 60. Relay 50 at the backcontacts of its armatures 53 and b t connects the receiving branch 23 ofthe main line to the terminals of the recording magnet 23 and at theback contact of its armature as it closes the driving circuit for themotor 2'5 to move the tape 38. This places the tape 30 in condition toreceive speech wavesfrom the distant terminal of the line IVE... Thus,during this half of the cycle of interrupter 52. tape l8 transmits itsrecorded material to the line ML and tape 38 records material receivedfrom the line ML.

Following through the operation of the receiving side of the channel,the portion of the receiving tape shown at 56 is being continuouslydriven by a motor (not shown) which is operating at all times to movethe tape portion 56 one half as fast as the tape portion 30 moves duringits time of movement. The waves picked off from tape portion 56 by thereading magnet 51 are seen therefore to be of the same general characteras the waves that are received in the receiving branch 23 of Fig. 3. Therectifier 5B and filter 59 operate in the same way as rectifier 3| andfilter 32 of Fig. 3 to derive the fundamental frequency which is thenused to drive the i motor 6| by an intermittent motion (as indicated bythe impulses shown at El) when the relay til is energized. When there isno voiced component from which to derive the fundamental frequency,relay 60 is deenergized and connects local generator 62 to the rectifier25 and motor 6! to drive the latter intermittently at anaveragefundamental rate. A portionof the waves picked up by readingmagnet 51 is impressed on recorder 64 which records on the continuouslyrunning tape portion 63 speech segments of the type of those recorded ontape portion is in the transmitting circuit. In other words, the recordmade on tape portion 63 consists of speech segments of one half thefundamental length with no blank spaces between. These recorded speechsegments are read by the reading magnet 66 from tape portion 65 movingintermittently in such a way as to transmit each recorded segment to theline L1 with intervening silent intervals corresponding to the waveportions deleted at the transmitter.

During the half period of interrupter 52 when the brush is on theconducting segment relays 50 and 5! are energized. Relay 5i opens thedriving circuit of motor i9 causing the tape por tion 3 to stop andallowing the tape portion 85 to supply slack tape preparatory to asubsequent movement of the tape portion 18. Relay 5i at its lowerarmature closes the driving circuit for motor i9 which starts movementof tape it, permitting the sending of stored speech segments fromreading magnet 20' through amplifier 2i to-the main line ML. Relay 5% atthe back contacts of its armatures 53 and 56 has now broken the circuitleading from receiving branch tions move at corresponding speeds. Thecorrect phase adjustment of the interrupters for an particularinstallation may be determined by trial. The period of the-interrupter52 may be as long as desired but for purposes of two-way talking itshould not be long enough to interfere with the carrying on of ordinaryconversation including replies which one talker makes to the other.Since a single speech segment re- 10 corded on the tape 65, for example,is of the masking noise to fill in the idle times correspond- 23 to therecording magnet 29 and at the front 55 contacts of its armatures 53 and54 has connected branch 23 to the terminals of recording magnet 29'.Relay by its armature 55 has opened the driving circuit for motor 21 andhas closed the driving circuit for motor 27'. The operation of sendingfrom and receiving by channel 2 is the same as that above described forchannel I, the corresponding apparatus elements being designated by thesame reference characters primed in the case of channel 2. It will beunderstood that the apparatus inside the broken rectangle labeledchannel 2 is identically the same as the apparatus inside the brokenline enclosure designated channel 5.

' It will be understood that the interrupter 52 must be run insynchronism with the corresponding interrupter at the distant stationand that the tapes as and 3d and i8 and 30' runat the same speeds as thecorresponding tapes at the distant station and that the other tapeporing to the segments of speech of fundamental length that are droppedout. Speech waves re-.

ceived from the line L3 are analyzed for their fundamental frequency asin the previous figures by rectifier l0 and filter H. Relay i2 isenergized when voiced waves are received and deenergized in the absenceof voiced waves. Relay H is vibrated under control of the fundamentalspeech frequency when relay [2 is energized and under control ofgenerator M when relay I2 is deenergized. Relay H connects alternatelynoise source l0 and the output of amplifier l6 directly to thetransmitting side of line L4.

The manner in which these waves are received at the distant station maybe seen from considering waves received from the line L; at the stationshown. The received waves are analyzed for the fundamental period of thespeech by rectifier 58 and filter 59 as in the previous figure and therelay H! is caused to vibrate its'armatures under control of thefundamental frequency when it is present and undercontrol of localgenerator 62 when the fundamental frequency is not present as determinedby the relay 60. It

is seen that with a proper control of the timing of the armatures ofrelay TI the receiving branch of line L4 may be connected to thereceiving amplifier 18 at those particular instants of time when thedesired speech segments are being received but disconnected from theamplifier 18 during the times when fragments of the masking noise arebeing received. In this way the speech segments are separated from thenoise segments and are transmitted to the line L3. Obviously :the localgenerators l4 and 62 should run in synchronism with and in proper phaserelation relative to the corresponding generators at the distant end ofthe line. The effect of listening on the line L4 is that of hearing themasking noise .from the source 10 without hearing the speech from theline L3, since the higher amplitude masking noise overpowers the desiredspeech sounds.

As an alternative a second conversation may ,be substituted for themasking noise where privacy for this second conversation is notrequired. In such case the back contacts of the armatures of relay 11would be connected to a second receiving branch for receiving thesecond, non-pri- .vate speech.

Referring back to Fig. 4 it is interesting to observe the frequency andword rate involved in transmission with that circuit. Since lines L1 andL2 both transmit intelligence over multiplex -line ML, but thetransmission takes place from asses-re lines L; and is one at a time, itis evident that during the time of transmission from either line, sayLathe rate of transmission of information over line m. is twice the rateon line L1 for the particular information considered. This means thatthe word rate is doubled. As was pointed out, however, the frequencyrange is not increased over that used on line L1. Fig. 4, therefore,exhibits a method of increasing the word rate, as by a factor of 2,without changing the frequency range. This is done by eliminating a partof each fundamental speech fragment, such as half, and transmitting theretained portions in continuous succession. In Fig. 4 this involvesmoving tape l8 continuously past reading magnet 28 at the. rate ofmovement of tape I5 during its times of movement.

Figs. 6, 6A and 7 illustrate how use may be made of this principle inrecording and reproducing speech. In Fig. 6 speech spoken intotransmitter 80 is first recorded on a tape 8| driven intermittently justas in the case of tape I5 of Fig. 3 or 4. The speech waves pass intosubdivider circuit 9 and the retained fragments are recorded at IT. Inthis figure the motor 88 is arranged to have its speed readily adjustedto suit different conditions and is therefore shown as driven from abattery under control of relay 85 which is caused to vibrate itsarmature at the fundamental frequency as indicated by the impulsediagram 85'. If variable resistance 86 is infinitely large, the motor isstarted and stopped as in Fig. 3 or 4 to drive the tape 8| forward injumps. The speed of rotation of the motor is controlled by resistor 81.The tape is wound from reel 82 to reel 83 and a record is prepared of aspeech, dictation or other spoken material.

The tape is then rewound on spool 82 and is afterward run past thereading magnet 98, Fig. 7, which is connected to telephones 9| or to aloud-speaker or other output device. The motion of tape 8| in thereproducing process is continuous and the speed is controlled byadjusting variable resistance 93 in the circuit of motor 92. If thespeed is the same as that used in recording during the times of movementof the recording tape, and if no spaces are left between the recordedsegments on the record tape, the word rate is doubled, but theintelligibility is substantially unaffected because the frequencyenergydistributionor spectrum, on which intelligibility depends, remainsunchanged. The fundamental frequency, of course, is also doubled in thisparticular case and this has the effect of shifting the register, aneffect similar to that produced when a man talks in a false upperregister simulating a woman's voice. This does not materially alter theintelligibility however. It is thus possible to play back the recordedmaterial at a greatly increased talking rate, if desired, withoutsacrifice of intelligibility. This would not be possible with ordinaryrecording such as is used in dictating machines and gramophones, for anincrease in record speed-above normal for reproducing purposes quicklydestroys the intelligibility since it alters the frequency energydistribution or spectrum.

The apparatus shown in Figs. 6 and 7 enables various ratios to beobtained between spoken word rate and the rate of reproduction. It hasalready been pointed out that if half of each fundamental period isdropped out and half retained, the word rate can be doubled withoutaltering the spectrum distribution. If instead rollers as in Fig. 6, aclutch is used operated by a cam as is done in Fig. 4 of my prior Patent2,115,803 referred to, the cam may be so shaped as to cause movement ofthe recording tape in smaller jumps than correspond to one half thefundamental speech period, resulting in dropping out more than half thefundamental length segment of the voice wave and retaining less thanhalf. Fig. 6A shows how such a cam may be used in a simple modificationof the apparatus of Fig. 6. Motor 85 drives roller 96 at a constantspeed but Wire 8| is not moved until idler 91 is depressed to engage thewire 8| between roller 96 and idler 91. Idler 91 is carried on a lever98 flexibly mounted on bed-piece 99. Cam |0| driven from motor 88engages cam i110 and depresses the lever for a time which can bepredetermined by the contour of the cam |0|.

If the fraction of the fundamental length speech fragment that isrecorded is one third, the word rate can be multipled by three withoutchanging the sound spectrum upon reproduction. Any other multiplicationratio can be obtained, such as 2.5, 3.1, 1.01, etc., by making thedesired ratio the same as the ratio of the fundamental lengthspeechsegment to the fractional part of such segment that is retained, theremainder of each segment being deleted.

A decrease in word rate can be obtained by the apparatus shown in Figs.6 and '7 by adjusting resistors 86, 81 and 93 as now to be described. Ifmotor 88 is allowed todrive the tape 8| forward in the non-recordingtimes, that is, when relay 26 in the subdivider circuit is shunting outthe recorder (see Fig. 3), blank spaces will exist on the record tape 8|between the segments of recorded material. Let it be supposed thatresistors 86 and 81 are given such values that when relay 85 isenergized tape 8| is driven at one rate and when the relay isdeenergized the tape is driven at some faster rate. The

record then consists of'segments of waves of,

of directly connecting motor 88 to the driving it say, half fundamentallength separated by blanks of greater length. If the blanks were alsohalf .fundamental length, there would be no change in either word rateor frequency distribution when the record is played back at the recordspeed. But if the blanks are longer than the half fundamental length (inthis case) there will be a decrease in word rate upon reproducing thesounds but no change in frequency distribution assuming the record ismoved at the recording speed. This is seen from the fact that if thesound ah were spoken into transmitter 8!] with a duration of one-thirdsecond, and if the record of this sound consisted of half periods offundamental length separated by blank spaces of fundamental length, itwould require 50 per cent longer time to reproduce-this sound from therecord with the speed of movement which the recording tape had in therecording intervals. In other words, the word rate is decreased in theratio of 3/2, without altering the frequency distribution. As a generalrule, if the speed'of the tape in the non-recording intervals, in orderto make blank spaces in the record, is M times faster than the speedduring the recording times, the word rate upon reproducing the sounds toretain the same frequency distribution is reduced in the ratio M 1 Theinvention is not to be construed as limited to the specific featuresdisclosed, these being given as illustrative embodiments. The scope isdefined by the claims which follow.

What is claimed is:

l. The method comprising subdividing speech waves on a time basis at thefundamental vocal cord frequency, dropping out a fractional part only ofeach subdivided wave portion of fundamental length, and transmittingonly the retained parts of such subdivided wave portions.

2. The method comprising subdividing speech waves on a time basis at thefundamental vocal cord frequency, dropping out a fractional part only ofeach subdivided wave portion of fundamental length, thus leaving idleintervals between the retained parts of successive subdivided waveportions, and filling in said idle intervals with fragments of a secondwave.

3. The method comprising recording on a suitable record medium a partonly of each fundamental period wave segment of a speech wave. andreproducing the recorded waves from said record with the time relationbetween the successive reproduced parts different from the time relationexisting between such parts in the recording process. 7

4. The method of speech transmission comprising subdividing speech waveson a time basis such that each speech period corresponding to thefundamental speech frequency is divided into two portions, suppressingthe lowenergy portion of each such period and transmitting only theremaining portion.

5. The method of speech transmission to increase the use of giventransmission facility comprising chopping speech on a time basis so asto divide each speech period of fundamental length into two parts,eliminating part of each such fundamental length speech period, storingthe retained portions in continuity with one another and transmittingsaid stored portions with said continuity.

6. The method of transmitting speech with reduced frequency rangecomprising chopping the speech on a time basis so as to divide eachspeech period of fundamental length into two parts, eliminating part ofeach such fundamental length speech' period and increasing thetransmission time of the remaining parts to use the time normallyoccupied by the eliminated part.

7. The method of treating speech waves comprising subdividing said wavesat the fundamental vocal cord vibration rate such that the voiced soundsexhibit in the case of each subdivision a high amplitude portion rapidlyclamped to much smaller amplitudes and selectively utilizing for soundeffects only the relatively high amplitude portion of the waves in eachsuch subdivision in the case of voiced sounds and a correspondingportion of the waves in each such subdivision of the unvoiced sounds.

8. The method of transmitting speech with an altered word rate but withthe same frequency range comprising subdividing the speech Waves on atime basis at the fundamental vocal cord frequency into fragments offundamental period duration, recording only a fractional part of eachsuch fragment in succession with a certain space relation-on the recordexisting between the parts so recorded, and reproducing the recordedparts with the reproduction rate the same as the recording rate in thecase of each recorded fractional wave part but with the timerelation'between the reproduced parts different from the time relationoccurring between the same parts in the recording process, thedifference in said time relations being determined by said spacerelation on the record.

9. In a communication system, means to subdivide each of a plurality ofspeech waves into fragments at the fundamental vocal cord frequency ofthe respective speech wave, means to transmit a part only of each suchwave fragment including only the high energy portion of the fragment inthe case of the voiced sounds, thereby leaving between the transmittedfragments certain time intervals unused for purposes of transmission inthe case of each wave, such unused time intervals corresponding to theportions of the wave fragments not transmitted, and means for utilizingsuch certain time intervals resulting from the transmission of one ofsaid speech waves for the transmission of another of said speech waves.

10. In a communication system, means to subdivide each of a plurality ofspeech waves into fragments at the fundamental vocal cord frequency ofthe respective speech wave, means to transmit a part only of each suchwave fragment, including means to transmit said parts of said wavefragments at reduced rate, thereby reducing the frequency range requiredfor transmission, and means for utilizing the frequency range madeavailable by thus reducing the fre quency range required for thetransmission of one of said speech waves, for transmitting another ofsaid speech waves.

11. The method of altering the word rate of speech while retaining theintelligibility, said speech exhibiting in the case of the voiced soundswave portions recurring at the fundamental vocal cord frequency eachwith a high amplitude part and a part of much lower amplitude, whichmethod comprises recording only the high amplitude part of each suchrecurrent wave portion and a corresponding part only of the unvoicedwaves, leaving unrecorded the low am plitude part of each recurrent waveportion in the case of voiced sounds and a corresponding part of theunvoiced sounds, using one rate of record travel during the recordingperiods and an arbitrarily different rate, including zero, during thenon-recording periods whereby the space relations existing 011' therecord between recorded and unrecorded wave parts is different from thetime relations existing between the corresponding parts of the soundwaves as spoken, and reproducing the speech from said record movingcontinuously at substantially its rate of movement during the recordingintervals.

12. A method according to claim 11 including moving the record fasterduring the non-recording intervals than during the recording intervalswhereby upon reproducing rrom the record the word rate is decreased. 4

13. A method according to claim 11 including stopping the movement ofthe record in each non-recording interval whereby upon reproducing fromthe record the word rate is increased.

14. In combination, a source of speech waves, means to subdivide saidWaves on a time basis at the fundamental vocal cord rate into wavefragments of fundamental period length, a path for the transmission ofsuch waves, means to transmit to said path a. portion only of each suchwave fragment, and translating means connected to said path.

15. A combination according to claim 14 including a second source ofwaves of speech frequency range, and means for transmitting to said pathportions of waves from said second source in the intervals betweentransmission of said wave fragments from said first-mentioned source.

16. A privacy system for telephony comprising means to chopspeech-representing waves at their fundamental frequency, a path for thetransmission of said waves, means for transmitting to said path only aportion of each segment of the waves resulting from such choppingprocess, a source of confusion waves, and means to' transmit bits ofsaid confusion waves to said ath in the intervals between saidtransmitted wave portions.

waves, a telephone receiving instrument, a switch for connecting saidpath to said instrument and disconnecting it therefrom intermittently atthe rate of chopping employed in transmitting and in such phase as todirect the portions of the speech-representing waves into saidinstrument and exclude said confusion waves therefrom.

18. A privacysystem according to claim 16 in which said confusion wavescomprise intelligence-bearing waves, receiving means, and means at areceiving location on said path for intermittently switching said pathinto operative relation with said receiving means in timed relation withthe chopping of said speech-representing waves and in such phase as toactuate said receiving means substantially exclusively with saidconfusing waves.

HOMER W. DUDLEY.

